This invention relates to processes for the production of vinyl fluoride, and more particularly, to catalysts and catalytic processes for the dehydrofluorination of 1,1-difluoroethane to vinyl fluoride.
Vinyl fluoride (i.e., CH2=CHF or VF) is a useful monomer for the preparation of fluorocarbon polymers which have excellent weathering and chemical resistance properties.
Vinyl fluoride can be produced from acetylene and hydrogen fluoride using mercury catalysts. It can also be produced by the dehydrofluorination of 1,1-difuloroethane (i.e., CHF2CH3 or HFC-152a). U. S. Pat. No.2,892,000 discloses a process for the manufacture of vinyl fluoride and 1,1-difluoroethane. In this process HF and acetylene are passed over a chromium catalyst (e.g., a chromium oxide or chromium salt catalyst) to obtain mixtures of VF and HFC-152a. A process is also disclosed in this patent for the conversion of product HFC-152a to VF using these catalysts. The patent describes using for this conversion of HFC-152a , catalysts which have been used for the reaction of HF and acetylene and whose activity has been reduced; and have then been treated by passing air or oxygen over the heated catalyst (e.g., at about 600 to 700xc2x0 C. for 1 to 3 hours) whereby it is activated for use in the dehydrofluorination of the HFC-152a to VF (e.g., at temperatures of from about 200xc2x0 C. to 400xc2x0 C. at a rate of about 20 to about 80 volumes per volume of catalyst per hour). There is an ongoing interest in developing more efficient catalysts for the conversion of HFC-152a to VF.
This invention provides advantageous processes for the production of vinyl fluoride and advantageous methods which may be employed for the preparation of catalysts useful in such processes. A method is provided to reduce surface B2O3 present in a bulk chromium oxide composition containing surface B2O3. The method comprises contacting said bulk chromium oxide composition with HF at an elevated temperature (e.g., from 200 to 400xc2x0 C.). Also provided is a method for treating a bulk chromium oxide composition containing B2O3 to enrich the B2O3 present on its surface. This method comprises heating said composition in oxygen or an oxygen-containing environment (e.g., air) at an elevated temperature for a time sufficient to enrich the B2O3 on the surface of the composition by at least a factor of two compared to the surface B2O3 content of the untreated bulk composition. Further provided is a method to reduce the amount of B2O3 present in a bulk chromium oxide composition containing B2O3 by first treating the bulk chromium oxide composition to enrich B2O3 present on its surface as indicated above, and contacting the surface-enriched composition with HF in the vapor-phase at an elevated temperature.
Processes are provided herein which comprise contacting 1,1-difluoroethane in the vapor phase with a trivalent chromium catalyst (preferably a trivalent chromium catalyst having primarily the morphology of alpha-chromium oxide and/or containing less than 1000 ppm alkali metal as the alkali metal oxide) wherein chromium is at least 95 atom percent of the metallic cations of said catalyst, at a temperature between about 225xc2x0 C. and 375xc2x0 C. Advantageous embodiments of these processes are provided wherein the catalyst is prepared by reducing B2O3 present in a bulk chromium oxide composition as indicated above. Further advantageous embodiments are provided wherein chromium is at least 99 atom percent of the metallic cations of the catalyst. Also provided are advantageous embodiments wherein the space velocity is from about 200 volumes to 2000 volumes of 1,1-difluoroethane per volume of catalyst per hour.